Human Heart
 

[CENTER][B]Heart[/B][/CENTER]

Heart is a hollow muscular organ that pumps blood to all parts of the body. The circulatory system is one of the vital systems of the human body and is responsible for distributing oxygen and nutrients to the cells and tissues of the body and carrying away carbon dioxide and other waste products. The circulatory system is made up of the heart, blood and various blood vessels. Circulation is the process by which nutrients, respiratory gases, and metabolic products are transported throughout a living organism, permitting integration among the various tissues. The process includes the intake of metabolic materials, the conveyance of these materials throughout the organism, and the return of harmful by- products to the environment. The heart is the main component of the circulatory system and is considered as its power supply.

The heart must function ceaselessly and continuously because the body's tissues depend on a constant supply of oxygen and nutrients delivered by the flowing blood. If the heart stops pumping blood for more than a few minutes, death will result. The tissues that are affected the most by any fluctuations in the supply of blood are the brain, nerves and the heart muscle itself. If the brain doesn't get a proper supply of blood, death of brain cells results, leading to death of the person. Lowered amounts of blood supply to the heart muscle itself would lower the ability of the heart to pump blood to all parts of the body. This would cause brain damage and death of the person. The human heart is shaped like an upside-down pear and is located slightly to the left of center inside the chest cavity.

The heart consists mostly of muscle, the myocardial cells arranged in ways that set it apart from other types of muscle. The outstanding characteristics of the action of the heart are its contractility, which is the basis for its pumping action, and the rhythmicity of the contraction. This periodic contraction begins in the developing embryo about three weeks after conception and continues throughout the lifetime. Heart muscle differs from its counterpart, skeletal muscle, in that it exhibits rhythmic contractions. The amount of blood pumped by the heart per minute i.e. the cardiac output varies to meet the metabolic needs of the peripheral tissues like muscle, kidney, brain, skin, liver, heart, gastrointestinal tract. The cardiac muscle rests only for a fraction of a second between beats. During the typical life span of a human being, the heart will beat nearly 2.8 billion times and pump169 million liters of blood.

Since prehistoric times people have had a sense of the heart?s vital importance. Cave paintings from 20,000 years ago depict a stylized heart inside the outline of hunted animals such as bison and elephant. The ancient Greeks believed the heart was the seat of intelligence. From the time of Aristotle in the 4th century BC it had been widely believed that the blood vessels contained both blood and air. Galen, the Greco-Roman physician, in the 2nd century AD proved that the arteries contained only blood but still believed that air entered the right side of the heart from the lungs.

Early in the 16th century the idea of a pulmonary circulation?that is, a circular motion of blood through heart and lungs?began to occur to some anatomists. It was the English physician William Harvey who discovered the true nature of the circulation of the blood and of the function of the heart as a pump. Other cultures had their own beliefs regarding the importance of the heart. Many of them believed the heart to be the source of the soul or of the emotions. This idea persists even today in popular culture and various verbal expressions, such as ?heartbreak.? Heart is also considered as a symbol of love and affection.

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[CENTER]CIRCULATORY SYSTEM[/CENTER]

Circulatory system or cardiovascular system is a system that combines the functions of the heart, blood and blood vessels to supply the organs and tissues of the body with oxygen and nutrients and carry away waste products. Basically, the circulatory system is an organ system that moves substances to and from cells. The circulatory system increases the flow of blood to meet the increased energy demands during exercise. It also helps in maintaining a constant body temperature and pH. It is responsible for maintaining homeostasis. The circulatory system also fights against foreign bodies which cause infection. When foreign substances and disease causing organisms invade the body, the circulatory system swiftly transports white blood cells and antibodies to the region of infection. White blood cells and antibodies are the disease fighting elements of the immune system. In case of injury or bleeding, the circulatory system sends clotting cells and proteins to the injured site. Clotting cells quickly stop the bleeding and promote healing.

The heart is the most vital component of the circulatory system. It has four chambers namely: the right atrium or right auricle, the right ventricle, the left atrium or the left auricle and the left ventricle. The walls of these chambers are made up of myocardium which contacts continuously in a rhythmic manner to pump blood to all parts of the body. The pumping action occurs in two stages for each heart beat. When in resting stage, it is called diastole and when the heart contracts to pump deoxygenated blood towards the lungs and oxygenated blood to the body, it is called systole. About 60-90 ml of blood is pumped out of the heart during each heart beat.

Blood is made of three types of cells: red blood cells that transport oxygen, white blood cells that fight against disease causing agents, and the platelets which are responsible for clotting of blood. All the three types of cells are carried through blood vessels in liquid called plasma. Plasma is yellowish in color and is made up of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats. There are three types of blood vessels in the body. These form a complicated network of tubes throughout the whole of the human body. The arteries and veins are joined together by means of capillaries. Capillaries are the tiny links between the arteries and the veins where oxygen and nutrients diffuse to the cells and tissues of the body. Endothelial cells line the inner surfaces of the blood vessels. They help in creating a smooth passage for the flow of blood through the blood vessels. Blood vessels can expand and contract in accordance with the passage of the blood because they are lined by elastic muscular tissue. During rigorous exercise, blood vessels expand to meet the increased demand for blood. Expansion of blood vessels also helps in cooling the body. Blood vessels also contract as and when required. For example, blood vessels contract after an injury to reduce blood loss and also to conserve body heat.

The walls of arteries are thicker than those of veins. The reason behind this is that arteries have to resist the pressure of blood pumped through them by the heart. Veins have one-way valves to prevent back flow of blood. Capillaries are the smallest blood vessels. Being of microscopic dimensions, they are only visible by microscope.

There are two systems of circulation namely systemic and pulmonary circulation systems. The systemic circulation transports oxygenated blood from the heart to all parts of the body except the lungs and returns deoxygenated blood to the heart. The pulmonary circulation carries this spent blood from the heart to the lungs. In the lungs, the blood releases its carbon dioxide and gets oxygenated. This oxygenated blood then returns to the heart before being transferred to the systemic circulation